Pressure sensors are used in pumps to measure water pressurization and as part of monitoring and control systems. Typically, an off-the-shelf pressure sensor will not survive application-specific operating conditions. Environmental conditions can also cause standard pressure sensors to fail. To ensure that pressure sensors maintain the operating efficiency of water pumps while minimizing service calls, several factors should be considered when specifying a sensor for pump systems in water service.
Media Compatible Materials
Pressure sensors used in pumps will contact water with varying pH levels and quality depending on water purification processes and added chemicals. Many new oil and gas fields also produce some levels of hydrogen sulfide (H2S) and salt water that must be pumped and stored in tanks prior to discharge. Pressure sensors must be able to work reliably in these elements without failure.
Units constructed from standard materials using austenitic steels (including 304 and 316) and ferritic steels (such as 13-8PH, 15-4PH and 17-4PH) can handle all types of water. However, these materials may not be sufficient to prevent common corrosion—such as pitting, crevices, stress cracking, intergranular corrosion and galvanic corrosion.
The corrosion rate depends on the liquid temperature, introduction of oxygen and water type. Nickel- and cobalt-based superalloys help maintain system reliability and safety because they resist corrosive acids. For example, a special nickel-based alloy provides excellent resistance against pitting and attack by micro organisms, enabling pressure sensors to be specified for use in shallow and warm waters with high levels of oxygen. Titanium and a nickel-molybdenum-chromium alloy give pressure sensor technology greater resistance to pressure and corrosion in water depths down to 7,500 feet and with an external pressure of approximately 3,800 psi. See Table 1 for material recommendations.
Pressure sensor end users must ask the manufacturer to state the sensor materials of construction of the assembly and the material’s compatibility to avoid costly failures in the field. Pressure sensors constructed from a one-piece 316L stainless steel sensing element also offer excellent liquid compatibility for residential and commercial water applications. A one-piece design ensures that outside media does not permeate the sensor body.
Depending on the pump system’s location, pressure sensors may be exposed to climate extremes—such as in Canada and Alaska, where the temperature often falls to -60 F (-50 C), or in Texas, where they will experience extreme heat and humidity. Pressure sensor threads must seal the media from low to high temperatures with a dry seal—such as national pipe taper (NPT) or a metal-to-metal conical seal. NPT threads can be used up to 10,000 psi (700 bar). However, caution must be taken against incorrect installation torque. Beyond 10,000 psi, an NPT thread must be avoided, and the units must be specified with a metal-to-metal seal.
Pressure transducers used on outdoor applications must withstand ambient temperature and rain, snow, icing and dirt. Corona discharge from lightning strikes and electromagnetic interference from electrical polluters—such as wireless radio systems, valves and variable drive motor systems—can cause stray voltages and leakage currents to be transmitted through pipes. This can often disrupt sensor operation. Pressure sensors with custom electronics and a sensing element to withstand 500 volts of direct current isolation work best in these extreme electrical conditions (see Image 1).
Rain, ice, dust and pressure washers can cause water to seep into sensor housings, causing electronics to short. Sealed gauge reference pressure sensors protect electronics from these conditions. Ingress Protection (IP) ratings can help end users understand the environment in which the sensor will survive—including high pressure, washdown or submersion. In certain cases, a submersible pressure transducer will be specified. The area in which the sensor is threaded and installed could become flooded, requiring that the electronics be protected from submersion.
The opening and closing of system valves, such as solenoids, can cause a pressure transient within a water pump above the measurement range in a shockwave. An internal welded restrictor plug reduces the inside diameter of the sensor’s pressure port so that when the pressure spike hits the face of the plug, it feeds through the hole at a slower pace, dampening pressure spikes before reaching the sensor’s diaphragm. The restrictor plug, commonly called a snubber, also reduces the overall length, cost and potential leak points in the system.
In this configuration, the liquid must be free of dirt, lint, iron filings, sand and grit to avoid clogging the plug. For some pressure sensing technologies—such as thin film, bonded metal foil strain gauge and capacitive with very low signal output—this is the only way to limit the diaphragm travel. Therefore, the user must install a mesh filter in front of the snubber.
¼-inch NPT male with snubber, ½-inch NPT male.
With inorganic, bonded semiconductor technologies, inorganic materials are molecularly diffused onto a metallic surface in the presence of certain gases. The high level of raw output provides significant deranging of the sensing element, allowing the use of large cavities without clogging (see Image 2).
With deranging, pressure sensors with this 5,000 psi (350 bar) sensing element can maintain high, full-range accuracy from 0 to 1,500 psi (100 bar) and offer over-pressure capabilities of 10,000 psi (700 bar). This is useful in compression pumps, on hydraulic fracturing trucks, and during icing conditions on casing and tubing lines.
Depending on the distance and environment, certain output signals can experience signal loss or generate signal noise. Water pressure sensors can be packaged with high-level voltage, current and frequency outputs. A 4-20-milliampere output signal is recommended for transmission lengths of more than 15 feet in environments with electrical noise. Frequency output signals are also used by niche manufacturers to provide direct pump feedback into a variable frequency drive. With a signal of 1 to 6 kilohertz, pump manufacturers can save electronic component cost.